In X-ray fluorescence spectrometers, the chemical composition of a substance is determined by irradiating the sample under test with X-rays and by recording the secondary X-ray fluorescent radiation emitted by the sample. Resolution of the sample fluorescence into its component wavelengths is achieved with the aid of an analyzing cristal which may be flat (as in Soller spectrometers) or curved (as in Johann or Johansson spectrometers). In the latter case the analyzing cristal provides focusing of the radiation reflected thereby at the focal circle defined by the curve of the planes of the analyzing cristal.
In the X-ray spectrometers commonly used nowadays the sample is positioned at a considerable distance (from 20 to 40 mm) from the focus of the X-ray source (X-ray tube). Therefore, to ensure the required sensitivity, such spectrometers must employ powerful big-sized X-ray tubes.
As is well known, soft X-ray radiation (having a relatively long wavelength) is strongly absorbed by air, which makes it impossible to determine the concentrations of light chemical elements in the test sample unless some special measures are taken to ensure a small degree of absorption of soft X-ray radiation.
In the known X-ray fluorescence spectrometers this is achieved by installing the whole X-ray system, including the X-ray source and the sample-loading device, in a chamber which is evacuated or filled with a light gas (helium). Such spectrometers make possible determination of concentrations of both heavy and light chemical elements.
However, spectrometers with such an evacuated chamber do not permit analysis of the samples susceptible to destruction by vacuum, such as solutions, powders, vegetable substances, etc. This substantially narrows the range of substances which can be analyzed. The employment of a helium-filled chamber makes the spectrometer very cumbersome in construction and use.
Besides, in such spectrometers the sample is difficult of access, which increases the time required for analysis and therefore reduces the productivity of the spectrometer and makes the sample-loading device more complicated.
Furthermore, breaking of vacuum occurring in such spectrometers when a sample is installed or removed necessitates restoration of vacuum each time a sample is installed. This also leads to increase in the time required by analysis and, besides, makes necessary the use of powerful vacuum pumps, which considerably increases the size and weight of the spectrometer.
Known in the art is an X-ray spectrometer wherein the X-ray source is positioned fairly close to the surface of the sample. Such a spectrometer comprises an X-ray source, a sample holder positioned across the radiation path of the X-ray source and spaced from the X-ray source at a distance which ensures the illumination of the central portion of the sample surface not less than 0.3 ZU erg/s. cm.sup.2.w, where X is the atomic weight of material of the X-ray anode and U is the voltage across the X-ray source, in kilovolts, a curved analyzing cristal for focusing the fluorescent radiation of the sample, the curve of the planes of the analyzing cristal defining a focal circle, and a detector for recording the radiation reflected by the analyzing cristal, the sample holder being positioned so that the distance between the focal circle and the sample surface exposed to radiation does not exceed the product of the distance between the focus of the X-ray source and said sample surface by the ratio between the diameter of the focal circle and the length of the analyzing cristal (cf. U.S. Pat. No. 4,091,282).
To provide the above-mentioned illumination of the sample surface, the X-ray source must be located very close to the sample. The distance between the focal circle and the sample in such a spectroueter is also small. Thanks to the small distance between the X-ray source and the sample, such a spectrometer provides considerable increase in the aperture ratio of the spectrometer and thus makes it possible to drastically (several tens of times) reduce the amount of power consumed by the X-ray tube, and hence its size and weight.
However, in the absence of an evacuated chamber, such a spectrometer is not capable of determining the concentrations of light chemical elements, e.g. having atomic weight below 22, because of strong absorption of soft X-ray radiation by air.